Has Obamacare really signed up 10 million people?

As of Dec. 31, "more than 9 million people now have health care coverage because of the Affordable Care Act," writes Josh Marshall. "It now seems like the number is more like 10 million."

Here's how that number breaks down: 2.1 million people have signed up for private insurance through the exchanges. About 4.4 million people have signed up for Medicaid coverage. And about 3.1 million young adults got coverage through Obamacare's rule forcing insurers to cover dependents up to age 26. Then there's the unknown number of people who bought Obamacare-based coverage directly from insurers.

The question is how many of these people really got coverage through Obamacare?

The exchange numbers are solid. You can trust those. The 3.1 million young adults who got coverage through the new insurance regulations is a pretty reliable figure, too -- though it's measuring something different than what people are usually thinking of when they ask how many people Obamacare has signed up.

But the Medicaid numbers are more complex. Each state runs its own count, and the data includes people who enrolled in Obamacare's Medicaid expansion as well as people who were eligible under prior law. To the Obama administration's annoyance, some states are also counting people who're simply renewing existing Medicaid policies.

A lot of the 4.4 million people who got coverage under Medicaid in the last few months got it from Obamacare's Medicaid expansion. Even more would've gotten it if all the states were participating in Obamacare's Medicaid expansion. But some simply got coverage under the preexisting Medicaid program. (And then there are the 100,000+ people who thought they got Medicaid through Obamacare but, due to a bug, were never actually enrolled.)

A rough way to calculate how much of Medicaid's new enrollment is due to Obamacare would be to compare total Medicaid enrollment in November 2012 to total Medicaid enrollment in November 2013. But that's tricky to do: The federal government didn't begin collecting monthly data on Medicaid enrollment until October 2013. Individual states keep track but gathering their figures is tough.

Moreover, the federal government hasn't finished collecting December's Medicaid data. So while the exchange numbers are showing enrollment from Oct. 1 through Dec. 31, the Medicaid numbers are only showing December data from a few states. Full December data won't be released until the middle of January. Assuming Medicaid enrollment shows anything like the December spike that exchange enrollment showed, those numbers could easily send the total here to 13 or 14 million.

Which is all to say that Obamacare has gotten a lot of people health insurance -- but it'll be awhile till we know exactly how many. And it's worth remembering, too, that the big spike is expected to come in March, which is the last month people can sign up before they have to pay the individual mandate.